An internal combustion engine in a rear-wheel-drive vehicle with an automatic transmission supplies a torque to the rear wheels at all times when the transmission is in gear. The torque produced when a vehicle is stopped or moving at a very low speed is commonly known as creep torque. This creep torque purposefully causes the vehicle to accelerate when the driver's foot is not on the vehicle's brake pedal. If the driver does not want the vehicle to move, the driver may counter the creep torque with a braking force by pressing the brake pedal.
The inventors herein have recognized that, during cold weather at an initial engine start, the engine idle speed is high, say 1200 rotations per minute. High engine idle speed causes the transmission's torque converter to create a larger than usual creep torque, thereby driving the rear wheels. Furthermore, during these conditions the engine vacuum is low, because the engine is throttled less at the high engine speed and vacuum is consumed by the front end accessory drive, torque converter, catalyst heating, and cold engine friction needs. Thus brake booster vacuum can be low, a consequence further exacerbated by low atmospheric pressure at high altitude.
This combination of conditions may result in slippage of the driven wheels, such as the rear wheels of a rear wheel drive vehicle, if one or more of the rear wheels are on a low friction surface such as ice or snow. Suppose that the vehicle's driver applies enough braking force to stop the front wheels and the vehicle. From the driver's point of view, the driver is doing all that is required. If the rear wheels begin to spin, the tires lose their traction and the rear portion of the vehicle can slide sideways. The driver may either not know to arrest this with greater braking force or may be unable to arrest this with sufficient brake line pressure.
The inventors herein have recognized the above issue and have devised various approaches to address it. In particular, systems and methods for preventing or stopping wheel slippage on a stopped or nearly stopped vehicle are disclosed. In one example, a method, comprises: applying a brake force to all four wheels of a motor vehicle to stop the vehicle while torque continues to be applied to its driven wheels; and preventing wheel slippage of the driven wheels by reducing the applied torque when the vehicle is stopped or nearly stopped from moving in a forward direction. In this way, unintentional lateral movement of the rear portion of a vehicle due to creep torque can be prevented.
In another example, a method, comprises: applying a brake force to all four wheels of a motor vehicle to stop the vehicle while torque continues to be applied to its driven wheels; and in response to wheel slippage of the driven wheels when the vehicle is stopped from moving in a forward direction, reducing the applied torque to reduce the slippage. In this way, unintentional lateral movement of the rear portion of a vehicle due to creep torque can be stopped.
In another example, a system for controlling the driven wheels of a motor vehicle, comprises: a braking system coupled to the driven wheels and non-driven wheels of the vehicle, the braking system responsive to an operator controlled brake; an internal combustion engine coupled to the driven wheels through an automatic transmission; an air intake system including a throttle coupled to an intake of the engine, the throttle responsive to an operator controlled accelerator pedal; the automatic transmission having electrically actuated hydraulic clutches to engage one of a plurality of gears having different gear ratios, each of the gears when engaged coupling engine torque modified by the engaged gear to the driven wheels; and a controller controlling the engine and the transmission, the controller actuating a different one of the selected gears for each one of a range of predetermined speeds of the engine to provide a substantially constant torque applied to the driven wheels over the range of engine speeds when the throttle has been moved to an idle position and the braking system has slowed the vehicle below a predetermined speed or has stopped the vehicle. In this way, unintentional lateral movement of the rear portion of a vehicle due to creep torque may be prevented and stopped and a consistent creep torque may be applied at all times.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.